Semiconductor power devices such as power MOSFETs have been used to replace electromechanical relays in a variety of applications. Electromechanical relays are mechanical systems and therefore have a limited lifetime. In order to adequately replace the relays, the power devices must be able to withstand high voltages and carry large amounts of current. For example, in automotive applications, battery voltages are on the order of 12 volts but noise spikes within the automotive wiring harness can cause momentary voltage spikes up to 60 volts. In addition, automotive applications can often require current loads up to and exceeding 20 amps.
Power devices have been characterized with a figure of merit which can be written as: EQU Figure of merit=R.sub.DS(ON) .times.A
This figure of merit characterizes the product of the area of a device and the source drain resistance of a power device when the power device is conducting electricity and should be minimized. The units of the figure of merit are m.OMEGA..multidot.m.sup.2. The lower the figure of merit for a power device, the more current for a given area the device can conduct. State of the art devices with a breakdown voltage of 60 V have a figure of merit of about 1 m.OMEGA..multidot.m.sup.2.
Drift regions are included in power MOSFET devices between the drain contact and the channel region to enable the power MOSFET to conduct large amounts of current at large voltages. The figure of merit of a particular device can be related to the material properties used to construct the device and the drift region associated with the device. For example, due to the mobility, relative dielectric constant, and critical field associated with different substances, gallium arsenide will have about a 10 times lower figure of merit than silicon and silicon carbide will have about a 100 times lower figure of merit than silicon. However, gallium arsenide and silicon carbide are very expensive materials and cannot be used to efficiently create the remaining structures associated with the device. For example, gallium arsenide cannot be used to grow a thin gate oxide necessary for the operation of the power MOSFET.